This invention relates to a photodetector and more particularly to a photodetector circuit which provides both sensitivity and fast response time.
Photodetectors are commonly utilized to sense incident illumination and to produce a response indicating the presence of the illumination. Such photodetectors are utilized in numerous applications such as the detection of encoded patterns, electric eyes, switching devices and others. Various types of photodetectors are presently available including phototransistors, photosensitive field effect transistors and other types of photoresponsive semiconductor devices. In each case, a portion of the semiconductor device receives illumination, typically through a lens, and upon receiving a sufficient amount of illumination, current begins to conduct through the device.
One of the main problems in utilizing photodetectors is to achieve both high sensitivity as well as fast response time. In most situations, an immediate response of the photodetector is required in order to turn on appropriate circuitry, or to provide the necessary effect resulting from the presence of the illumination. As a result, a fast response time is desired to the receiving of illumination. This is especially so, when the photodetector is part of circuitry which is being utilized with computer technology wherein extremely fast speed is needed.
The high sensitivity is also of importance in order to discriminate between the presence of illumination amidst a background field. For example, in detecting encoded markings on a surface, the sensitivity is important in order to discriminate the encoding marks from the background noise. Similarly, a high sensitivity will make a more effective detector for such items as area detectors, scanning devices, etc.
One problem with prior art devices is that high sensitivity and high response times have been considered as incompatibly related features. Specifically, they were considered as being mutually contradictory, and the achievement on one was always at the expense of the other. For example, in order to obtain a high sensitivity of a photodetector, the response time was sacrificed. Conversely, in achieving a fast response time, the sensitivity of the photodetector was sacrificed.